An electrophotographic image forming apparatus forms a toner image corresponding to image information on a photoreceptor that is an electrostatic latent image carrier to transfer the toner image to a conveyed sheet by the action of a transfer electric field from the backside of the conveyed sheet, and is generally disposed with a sheet peeling means for peeling off from the photoreceptor the conveyed sheet stuck to the photoreceptor after transferring the toner image.
The peeling of the conveyed sheet in the image forming apparatus will specifically be described with reference to FIGS. 1 to 3. FIG. 1 depicts a configuration around a photoreceptor in the image forming apparatus, and the configuration around a drum-shaped photoreceptor 1 rotating in a direction of an arrow A of FIG. 1 includes a development device 2; a transfer charger 3 that applies the transfer electric field from the backside of the sheet; a sheet peeling charger 4 that applies an electric field with a reverse polarity of the electric field of the transfer charger 3 to enhance peeling property; a sheet peeling member 5 including a peeling claw 5a; a cleaning unit 6 including a cleaner blade 6a; a resist roller 7 for matching a leading edge of the sheet with the image information on the photoreceptor 1; paper guides 8, 9, etc., which are arranged along the rotation direction of the photoreceptor 1 from a position B where exposure laser light is emitted.
In the image forming apparatus with the above configuration, the toner image developed on the surface of the photoreceptor 1 is transferred at a point P1 of FIG. 1 to a sheet P conveyed, and the sheet P is stuck to the photoreceptor 1 due to the transfer electric field applied in this transfer process from the transfer charger 3 and is conveyed in the stuck state by the rotation of the photoreceptor 1. When the stuck sheet P is peeled off from the photoreceptor 1 by applying the electric field with the reverse polarity from the sheet peeling charger 4, the peeled sheet P drops onto the paper guide 9 from the leading edge due to the own weight and smoothly conveyed to the next process.
However, even if the electric field with the reverse polarity is applied by the sheet peeling charger 4, the sheet P is not peeled off from the photoreceptor 1 depending on the material of the sheet P and the image information, and the sheet P not peeled is conveyed while sticking to the photoreceptor 1 and the leading edge collides with the peeling claw 5a and is forcibly peeled off.
The sheet peeling member 5 is included for forcibly peeling off the sheet P that is not peeled off and is conveyed in the stuck state even if the electric field with the reverse polarity is applied by the sheet peeling charger 4; for example, as shown in FIG. 2A, an inclined surface X is disposed at a leading end of the peeling claw 5a; and an end of a star roller 5b is located on an extended line C of the inclined surface X such that the sheet peeled off by the leading end is smoothly separated from the photoreceptor (FIG. 2B).
Describing the mechanism of the sheet peeling with reference to FIG. 3, when the sheet P is stuck to and conveyed by the photoreceptor 1 and the leading edge of the sheet P reaches the leading end of the peeling claw 5a (step 1), the leading edge of the sheet P collides with the leading end of the peeling claw 5a and the leading edge portion of the sheet P is brought into a stripped-off state (step 2) and runs on the inclined surface X of the peeling claw 5a (step 3). When the sheet P is further conveyed by the rotation of the photoreceptor 1, the leading edge of the sheet P reaches the end of the star roller 5b (step 4) and, when the sheet P is further conveyed, the sheet P drops and is separated from the photoreceptor 1 due to the own weight (step 5).
By the way, if the developed toner image is transferred to the conveyed sheet with the use of the transfer electric field, a white portion (portion without image information) of the conveyed sheet receives the strong electric field from the transfer charger and is strongly stuck to the photoreceptor. Generally, in the case of a print sheet, the leading edge of the sheet often becomes the white portion and this portion is referred to as a “void area”. The void area has a function useful for preventing the entanglement jam of the sheet, which occurs if toner reaches the leading edge of the sheet when melting/fixing is performed for the sheet by a thermal fixing device after the transfer process is completed.
However, the void area often sticks to the photoreceptor as above and this is partially the reason why an electrophotographic image forming apparatus generally employs a configuration with the sheet peeling member including the peeling claw for returning the sticking sheet to a sheet conveying route.
If the sheet peeling member including the peeling claw is not disposed, the strongly sticking sheet P is not peeled off from the photoreceptor 1, is conveyed around the photoreceptor 1 while sticking to the photoreceptor 1, reaches the cleaning unit 6, a main charger (not shown), and the development device 2, which are arranged around the photoreceptor 1, to destroy the apparatus or to damage the photoreceptor surface, and causes deterioration of the print quality as well as reduction of the life property of each unit.
In technologies proposed for preventing the peeling jam from frequently occurring due to shape deformation or damage of the peeling claw leading end in the image forming apparatus disposed with the sheet peeling member, the peeling claw can be contacted with and separated from the photoreceptor depending on the presence of the conveyed sheet and the timing when the sheet leading edge reaches the peeling claw, as in Japanese Laid-Open Patent Publication Nos. H6-27753 and 2002-108110.
However, it is inevitable in any case that the peeling claw repeatedly collides with the sheet leading edge and that the leading end of the claw is deformed or damaged, and if the leading end is deformed or damaged, the sheet sticking to the photoreceptor cannot certainly be peeled off, which frequently causes the peeling jam.